Ms. Neuwirth, whose 10-year-old line is known for raw gemstones and intense colors, often carries loose stones to clients or ones she has stumbled on while traveling or out and about. So in addition to a small green Proenza Schouler bag, the Venice, Calif., designer has plastic bags in her bag for the stones. Celebrities who have worn her jewelry to big events in recent months include Maria Sharapova, Jessica Chastain...